An acoustic or phononic bandgap is the phononic analog of a photonic bandgap, wherein a range of acoustic frequencies are forbidden to exist in a structured material. Phononic bandgaps are realized by embedding periodic scatterers in a homogeneous host matrix that propagates an acoustic wave. The scatterer material has a density and/or elastic constant that is different than that of the matrix material, leading to destructive interference of the acoustic wave when the lattice constant of the phononic crystal structure is comparable to the wavelength of the acoustic wave. If the interference is destructive, the energy of the acoustic wave is reflected back and the wave cannot propagate through the phononic crystal. This destructive interference creates the phononic bandgap. The bandgap center frequency, spectral width (i.e., the range of frequencies over which phonons cannot be transmitted through the material), and the depth (i.e., the amount of acoustic rejection inside the bandgap frequency region) are determined by the size, periodicity, and arrangement of the scattering inclusions in the matrix material and the material properties of the inclusions and matrix. In principle, the bandgap can be created at any frequency or wavelength simply by changing the size of the unit cell of the crystal. The spectral width of the phononic bandgap is directly related to the ratio of the densities and sound velocities in the different materials comprising the structure. In general, the larger the ratio, the wider the bandgap. Further, for two- or three-dimensional phononic crystals, the frequency and width of the bandgap will depend on the direction of propagation.
Recently, bulk wave acoustic bandgap devices have been fabricated using microelectromechanical systems (MEMS) technologies. Phononic crystals have been fabricated at frequencies as high as 1 GHz, using high acoustic impedance scattering inclusions, such as tungsten, in a low acoustic impedance background matrix, such as silicon dioxide, and have been shown to block phonon propagation through a synthetic material over a wide frequency range. See U.S. patent application Ser. No. 11/748,832 to Olsson et al., which is incorporated herein by reference. At the micro-scale, these phononic crystals are useful for acoustic isolation of devices, such as resonators and gyroscopes. Furthermore, by strategically locating defects in the phononic crystal through removal or distortion of the scattering inclusions, micro-acoustic waveguides, focusing, sensors, cavities, filters, and advanced acoustic signal processors can be realized. These devices have applications in communications, ultrasound, sensing and non-destructive testing.
However, a need remains for phononic crystal devices that can be used in thermal management and noise mitigation. Therefore, a need remains to scale this technology to terahertz (THz) frequencies, the frequency range where most thermally generated room temperature phonons propagate.